This might be caused by a corrupted firmware on the telemetry radio. Re-install the firmware, as described in the next procedure:
Using mission planner:

  1. Connect the telemetry radio to the computer, select the serial port and 57600 bauds, don’t hit connect yet.
  2. Go to Initial setup >optional hardware >3DR Radio.
  3. Click upload firmware (local).
    APM planner doesn’t have an upload firmware option, but MAC users can try the next procedure in a terminal:

    1. wget
    2. wget
    3. sudo easy_install pexpect
    4. python –port ENTER_SERIAL_PORT_HERE(i.e. /dev/cu.usbserial-DN0019TF) –baudrate 57600 radio~hm_trp.ihx

    If the firmware upload doesn’t correct the problem then the telemetry radio is damaged or defective.